Liquid crystal display devices are often required to be adapted to a partial display in which an image is displayed on only a desired part of the display screen. For example, due to the recent trend of the increase in the screen size of portable terminals such as smartphones and tablets, a liquid crystal display device mounted on a portable terminal is often operated to display a desired image (such as, an image of a clock) only on a part of the display screen, when the portable terminal is placed in the standby state.
FIG. 1 is a schematic diagram illustrating an example of the partial display. When a portable terminal 100 including a liquid crystal display device is being operated by the user, that is, when the portable terminal 100 is in the “in-use” state, as illustrated in the left section of FIG. 1, the liquid crystal display device is placed in the full-screen display mode to display an image on the entire display screen. After a certain period of time has elapsed without any operation on the portable terminal 100, the portable terminal 100 moves to the standby state. In the standby state, a partial display, in which an image is displayed on only a part of the display screen of the liquid crystal display panel, is performed in the portable terminal 100. The center section of FIG. 1 illustrates an exemplary operation in which an image of a digital clock is displayed only on part A of the display screen of the liquid crystal display; no image is displayed on the remaining part. After a certain period of time has elapsed after the movement to the standby state, as illustrated in the right section of FIG. 1, the portable terminal 100 moves to the “not-in-use” state, in which no image is displayed on the entire display screen. When a certain operation (for example, a push of a certain button) is performed on the portable terminal 100 in the “not-in-use” state, the portable terminal 100 returns to the standby state. When a certain operation is then performed on the portable terminal 100 in the standby state (for example, when a certain “gesture” is performed on the display screen of the liquid crystal display device), the portable terminal 100 returns to the in-use state.
In general, a conventional liquid crystal display device is configured to drive all the subpixels of the entire display screen even when a partial display is performed. In detail, pixels in the part in which the image is not displayed are operated to display the black color, for example; the respective subpixels of each pixel are driven to display the black color. This is because a liquid crystal display device, in which the common electrode is driven to a specific common level, requires driving all the subpixels with an AC drive scheme to avoid damage to liquid crystal in the liquid crystal display panel. If the subpixels are not driven with an AC drive scheme, a DC voltage is applied between the common electrode and the pixel electrode of each subpixel, and this leads to damage to the liquid crystal.
The above-described conventional operation of a liquid crystal display device unnecessarily increases power consumption in a partial display. Reduction in the power consumption is one of the important requirements of high-resolution liquid crystal display devices for portable terminals, such as liquid crystal display devices mounted on recent smartphones and tablets.